US20150319818A1 - Circuit arrangement and led lamp comprising the same - Google Patents

Abstract

A circuit arrangement for operating at least one low-power lighting unit with a phase-cut operating voltage from a power supply is disclosed, which circuit arrangement comprises a serial setup of an input device (6), a two-port power shaping circuit (7, 7′, 7″) and a lamp driver unit (8, 8′, 8″). The power shaping circuit (7, 7′, 7″) comprises at least a bleeder (13) and a damping circuit (14). While the bleeder circuit (13) provides an alternative current path to set a global current, drawn during operation from the power supply to a predefined minimum load current, the damping circuit (14) serves to attenuate high frequency oscillations in said operating voltage. To enhance dimmer compatibility and simultaneously provide a cost-efficient circuit setup, a first and a second feedback circuit (18, 25) is provided, allowing to control the bleeder circuit (13) and the lamp driver unit (8, 8′, 8″) according to a two-point control.

Description

TECHNICAL FIELD
• The present invention relates to a circuit arrangement for operating at least one low-power lighting unit with a phase-cut operating voltage, a LED lamp comprising the same and a method of operating a low-power lighting unit.
BACKGROUND ART
• In the field of lighting, present efforts aim for reducing the power consumption in particular of residential and commercial lighting applications. Nowadays, lamps or light sources are being employed for the replacement of common incandescent or halogen lamps, which include one or more light emitting diodes (LEDs). LEDs feature a dramatically decreased power consumption in comparison to incandescent lamps with the same luminous flux and in addition provide a substantially increased lifetime. LEDs thus are very promising for new generation light sources.
• For retrofit applications however, i.e. as a replacement for incandescent or halogen lamps, LEDs typically cannot be used directly with common types of installed power supplies, but due to its exponential voltage-current behavior, require a dedicated driver circuit. The LED driver circuit usually adapts the voltage for the LED to the requisite level and also keeps the delivered current constant. A most simple “driver circuit” comprises a resistive element in series with the LED(s).
• A particular problem may arise from the fact that the reduced power consumption of LEDs results in an accordingly reduced operating current. In particular when using the LED lamp with a power supply having a phase-cut dimming unit, such as a leading-edge or trailing-edge dimming unit, the power supply may have a minimum load requirement, which may not be met by the LED lamp. Here, the reduced current may result in an unintended behavior of the dimmer/lamp combination, which may e.g. result in visible flicker.
• For example, since a trailing-edge (TE) type of dimmer typically is MOSFET-based and comprises an internal supply circuit which powers the timing and zero-crossing detection circuit, the “under-load” may result in that the internal supply cannot provide enough operating power to the timing circuit, causing problems with the zero-crossing detection of the sinusoidal mains voltage. In a leading-edge (LE) type of dimmer, a TRIAC or two anti-parallel connected thyristors are used, where the current typically needs to be high enough to maintain the TRIAC in a conductive state, i.e. above a holding current, so that the operation of a LED lamp with this type of power supply or dimmer may result in an “unintended” or untimely disconnect of the TRIAC(s).
• An object of the present invention therefore is to provide a circuit arrangement, which allows operating at least one low-power lighting unit with a phase-cut operating voltage, so that it is possible to operate said lighting unit with a variety of types of power supplies while maintaining high quality light output. A further object is to provide a cost-efficient circuit arrangement, which allows to be used for mass market applications.
DISCLOSURE OF INVENTION
• According to the present invention, the object is solved by a circuit arrangement, an LED lamp, a lighting system and a method of operating at least one low-power lighting unit with a phase-cut operating voltage according to the independent claims. The dependent claims relate to preferred embodiments of the invention.
• The basic idea of the present invention is to provide a circuit arrangement having a cascaded, i.e. serial multi-stage set-up to provide for a cost-efficient design while simultaneously provide high compatibility in particular to power supplies having dimmers.
• The circuit arrangement according to the invention comprises at least an input device, a power shaping circuit and a lamp driver unit in a serial connection.
• The power shaping circuit comprises a controllable bleeder circuit in series with a damping circuit. The bleeder circuit provides that the global current, i.e. the current drawn from the power supply during operation, corresponds to a predefined minimum load current to provide an enhanced compatibility to various types of power supplies and in particular such providing a phase-cut operating voltage, e.g. comprising a phase-cut dimmer. The damping circuit attenuates high-frequency oscillations, which may occur at each edge of the phase-cut operating voltage to allow high-quality light output.
• The lamp driver unit is configured to control a lamp current of at least one low-power lighting unit to a setpoint current based on a first feedback signal.
• Accordingly, the cascaded multi-stage setup allows on the one hand to keep the current provided to the lamp, i.e. the lamp current, constant while at the same time provide that the global current, drawn during operation on the power supply, corresponds to the predefined minimum load current, e.g. of the power supply. The present invention thus advantageously allows setting both, the input and the output current to the respectively desired setting independently from each other, enabling a two-point control.
• The inventive circuit arrangement is adapted for operating at least one low-power lighting unit with a phase-cut operating voltage from a power supply, as discussed before.
• The low-power lighting unit may be of any suitable type. Preferably, the low-power lighting unit is an LED unit comprising at least one light emitting diode (LED), which in terms of the present invention may be any type of solid state light source, such as an inorganic LED, organic LED or a solid state laser e.g. a laser diode. The LED unit may certainly comprise more than one of the before mentioned components connected in series and/or in parallel. The term “low-power” relates to the power consumption of the lighting unit compared to that of a conventional light source like an incandescent lamp. The power consumption of the at least one lighting unit is preferably below 20 W, more preferably below 15 W, most preferably below 10 W.
• The phase-cut operating voltage is a sinusoidal voltage, where a part of each wave/cycle (or usually each half-wave/half-cycle) is chopped or cut out. Starting from zero-crossing of the sinusoidal or alternating voltage, this may be the leading-edge part or the trailing-edge part.
• Although the power supply in this context usually comprises a “dimmer”, e.g. a phase-cut dimmer, sometimes also referred to as “phase firing controller”, in the sense that the part of the wave that is chopped—which corresponds to the timing of the phase-cut—can be adjusted by a user, it is also conceivable that this part is constant. Anyway, the time evolution of the voltage shows a comparably steep decline or rise on each phase-cut operation. Any phase-cut technology known in the art may be used in the context with the present invention. However, the inventive circuit is particularly suitable for use with a power supply having a leading-edge (LE) type of dimmer.
• As mentioned before, the inventive circuit arrangement comprises an input device, a two-port-power shaping circuit and a lamp driver unit, which are connected in a cascaded, i.e. serial three stage set-up.
• The input device is adapted for connection to the power supply, e.g. over suitable (detachable) connecting terminals, and comprises at least a first and a second supply (output) terminal. The two-port power shaping circuit comprises a first and a second input terminal and a first and a second output terminal. The output terminals are connected with said input terminals over a first and a second supply connection.
• Each of the before mentioned terminals may be connected by a permanent electrical connection, for example by soldering, or by a detachable connection, like a plug and a socket connection. The terminals should provide an electrically conductive connection at least in an operational state of the circuit arrangement.
• Any electrical connections mentioned in the context of the present invention may be switchable and furthermore may be indirect, i.e. comprising intermediate components, but are preferably direct.
• The power shaping circuit according to the invention comprises a controllable bleeder circuit, connected between the first and second input terminals to provide an alternative current path. As mentioned in the preceding, the bleeder circuit is configured to set the global current, drawn during operation from the power supply, to a predefined minimum load current.
• The controllable bleeder circuit thus allows setting the global current, i.e. the current drawn from the power supply through the first and second supply terminals of the input device, to the predefined minimum load current and independent from the further components of the circuit arrangement. In the present context, the bleeder circuit may provide that the global current corresponds at least to the minimum load current. Certainly, the global current may be set higher than the minimum load current. However a higher current may reduce the efficiency of the circuit arrangement.
• The predefined minimum load current may be set permanently in the bleeder circuit, provided by an external signal and/or may be controllable by a user using a correspondingly adapted user interface, a switch or a potentiometer for individual adaptation to the requirements of the respective power supply. The predefined minimum load current preferably corresponds to the minimum hold current of the power supply/dimmer, wherein the term “corresponds” includes a current setting, slightly higher than the minimum hold current, i.e. in a range of less than 15% higher than the minimum hold current of the dimmer. The minimum load current may be as high as 50 mA, but preferably is 20 mA, most preferred 22 mA and particularly preferred 35 mA.
• The controllable bleeder circuit may be of any suitable type. For example, the bleeder circuit may comprise a variable resistor to set the current between the first and second input terminals. Preferably, the bleeder circuit comprises a controllable current source or an adaptive current source. Herein, the term “adaptive current source” relates to a current source, where the amplitude/on-time of the current drawn is controlled in dependence of the dim level, the dim curve, lamp current and/or minimum load current. Most preferably, the bleeder circuit comprises a controllable clamp circuit to set the potential to ground potential in an off-state of the dimmer to allow a large current of approximately 200 mA. The bleeder circuit may comprise control circuitry of any suitable type, e.g. discrete and/or integrated electronic circuitry, and may comprise a microcontroller and/or one or more comparators.
• As mentioned above, the power shaping circuit furthermore comprises a damping circuit, connected to the first and the second supply connection of the power shaping circuit at a first and second connection point. Thus, the damping circuit is coupled to “intermediate” connection points in series with input and output terminals. The damping circuit is arranged to damp or attenuate high-frequency oscillations, i.e. typically in the range of 8 kHz-10 kHz for dimmers operated at 50 Hz mains frequency and between 10-100 kHz for dimmers operated at 60 Hz mains frequency, which may be present in the phase-cut operating voltage and in particular at the before mentioned dimmer edge. It is particularly important, that the damping circuit is arranged between the bleeder circuit and the output terminals, i.e. the lamp driver unit in the mentioned serial arrangement, so that the operation of the two circuits does not interfere with the respective other circuit.
• The damping circuit may be of any suitable type and preferably comprises a RC circuit. For example, the damping circuit may be a resistive/capacitive network, i.e. a combination of one or more resistor and capacitor. Preferably, the damping circuit is configured to draw an additional current from the power supply at or shortly after the phase-cut operation, i.e. the before mentioned steep decline or rise, caused by the phase-cut dimmer. Most preferably, the damping circuit is non-dissipative and comprises an energy storage device, such as a capacitor. Herein, the term “non-dissipative” is understood that the drawn current is substantially provided to further components of the circuit arrangement and in particular to a power supply and/or the lamp driver unit, e.g. at a different phase-angle or half-cycle of the phase-cut operating voltage.
• The inventive circuit arrangement further comprises the lamp driver unit, which is connected to at least one of the output terminals of the power shaping circuit and is configured for connection to the at least one low-power lighting unit. The lamp driver unit comprises at least a controllable lamp current controller, such as a controllable/adaptive current source, which is configured to control the lamp current of said at least low-power lighting unit. In the present context, the term “lamp current” is understood as the current flowing through the at least one low-power lighting unit in an operational state of the circuit arrangement.
• The lamp driver unit furthermore comprises a first feedback circuit, configured to provide a first feedback signal which corresponds to the momentary lamp current of the low-power lighting unit. The lamp current controller is connected with the feedback circuit to control the lamp current in dependence of the first feedback signal, so that the lamp current corresponds to a given setpoint current, i.e. to maintain the lamp current substantially constant (+/−0.5 mA ˜1%) in a closed-loop operation.
• The feedback circuit and the lamp current controller may be of any suitable type to determine the momentary lamp current and control the lamp current accordingly. The lamp current controller may comprise a control circuit, e.g. discrete and/or integrated circuitry, such as a micro-controller or a suitable set-up of one or more comparators. The feedback circuit may be formed by any suitable circuitry. Preferably, the feedback circuit is of analog type, i.e. the feedback circuit provides an analog signal, corresponding to the lamp current, allowing a cost-efficient setup of the circuit arrangement.
• Certainly, the feedback circuit may be formed integrated, e.g. as part of an analog and/or digital integrated circuit device (IC). The first feedback circuit may also be formed integrated with further components, e.g. the above mentioned lamp current controller. This may be particularly advantageous in case the lamp current controller is a switch mode power supply as discussed in the following. Here, the feedback circuit may be formed integrated with the IC that also controls the switch mode.
• The setpoint current may be predefined, e.g. in dependence of the respective type of low-power lighting unit connected or may be set externally, e.g. by a user or according to the respective dim level of the phase-cut operating voltage, which will be explained in detail further below.
• The lamp driver unit may be suitably adapted to control the current through the at least one low-power lighting unit, e.g. comprising one or more controllable current sources. For example, the lamp driver unit be a switch mode power supply, such as a buck, buck-boost, flyback of halfbridge converter. As will be apparent to one skilled in the art, a switch mode power supply circuit typically comprises a switching device and an energy storage, which is charged and discharged in cycles to adapt the voltage and/or current according to the application. The lamp driver unit in particular in this case may comprise a EMI filter circuit to attenuate high-frequency ripple, caused by the operation of the switching device of the switch mode power supply circuit, for example a PI filter (capacitor/inductor filter). The lamp driver unit may further comprise a buffer/fill-in stage, such as one or more suitably connected capacitors.
• Alternatively or additionally, the lamp driver unit may comprise a tapped linear driver, e.g. comprising multiple controllable current sources for the operation of an according number of low-power lighting units, e.g. LEDs.
• As discussed in the preceding, the lamp driver unit is connected to at least one of the output terminals of the power shaping circuit. The lamp driver unit may preferably be connected between one of the output terminals and a reference potential, such as ground potential. Certainly, the lamp driver unit may further preferred be connected to both of the output terminals of the power shaping circuit. In this case, it follows that the at least one low-power lighting unit is connected between the first and the second output terminals of the power shaping circuit.
• During operation of the inventive circuit arrangement, the lamp current is regulated by the current controller according to the desired setpoint current in a closed-loop operation. According to the inventive setup, this control is conducted in the lamp driver unit, i.e. the “third stage” of the circuit arrangement. Independently there from, the before mentioned bleeder circuit of the power shaping circuit, i.e. the “second stage”, maintains the global current at the desired minimum load current. The inventive circuit arrangement thus provides a “two-point” control in a cascaded or serial setup, which is particularly cost-efficient and allows improved control and compatibility in particular for low-cost mass market applications.
• Preferably, the bleeder circuit is configured to be activated only when the global current is lower than the predefined minimum load current. Since according to the above, the bleeder circuit may e.g. be of dissipative type, the present embodiment enhances the energy efficiency of the circuit arrangement further, since the bleeder is only activated, i.e. controlled to provide said current path between the input terminals, when the lamp driver unit and the at least one low-power lighting unit does not draw enough current to maintain the global current at the predefined minimum load current.
• Certainly, it should be mentioned that the damping circuit depending on its design may also draw a minor current during its operation, so that it should be understood that the bleeder circuit may be configured to be activated only when the sum of the lamp current and the current drawn by the damping circuit is below the minimum load current. However, the current, drawn by the damping circuit typically is negligible.
• As discussed above, the bleeder circuit may be of any suitable type to set the global current, drawn during operation from the power supply, to the predefined minimum load current.
• According to a preferred embodiment of the invention, the circuit arrangement, e.g. the power shaping circuit, further comprises a second feedback circuit, configured to determine a second feedback signal corresponding to the momentary global current and to provide set second feedback signal to the bleeder circuit.
• According to the present embodiment, a second feedback circuit or loop is provided to allow setting the global current corresponding to the predefined minimum load current during normal operation. Preferably, the second feedback circuit is also of analog type, i.e. the second feedback circuit provides an analog signal, corresponding to the global current. The setup of the first and second feedback circuit may be identical, e.g. of analog type.
• The present embodiment may be particularly advantageous when the circuit arrangement is used for low-cost lighting applications, since the provision of analog feedback circuits results in a very cost efficient design.
• As discussed above, the first and the second feedback circuit may be of any suitable type for determining the first and second feedback signals, corresponding to the momentary lamp current and the global current, respectively. The second feedback circuit may also be formed integrated as discussed above with reference to the first feedback circuit. In case that both the first and second feedback circuits are formed as integrated circuits, it is preferred that the feedback circuits are integrated in one IC.
• Most preferably, the bleeder circuit is configured to maintain the global current substantially constant in the range of 20 to 50 mA during most of the conduction interval, i.e. when using an LE dimmer, the time in each half cycle between the dimmer edge and the subsequent zero-crossing of the phase-cut operating voltage.
• According to a further preferred embodiment of the invention, the first and/or second feedback circuits are coupled to a series connection of a first and a second current sensing resistor, said series connection being coupled between said second supply terminal and a reference potential. Preferably, the reference potential is ground.
• As will be apparent to one skilled in the art, the voltage, determined at a sensing resistor, corresponds to the current flowing through it, so that the use of sense resistors allows providing a feedback signal with a particularly cost-efficient circuit setup. The provision of at least two sense resistors in series, which form a voltage divider circuit between the second supply terminal of the input device and the reference potential, is particularly advantageous for an efficient determination of the momentary global current and the momentary lamp current.
• During operation, the presence of the first and second sensing resistors “shifts” the voltage at the second supply terminal in respect to the reference potential. Thus, a current flows between the supply terminal and the reference potential through said first and second sensing resistors. This shift and the respective current depends on the current though the further components, e.g. the at least one low-power lighting unit.
• Advantageously, the present embodiment thus does not necessitate to determine the current at the lamp driver directly, where the presence of sensing resistors would cause power dissipation.
• The current sensing resistors may be of any suitable type and may comprise one or more resistive elements. The setup of the sensing resistors may comprise one or more zener diodes or transistors for adaptation of the voltage levels. The reference potential may be chosen according to the application. Preferably, the reference potential is a ground potential.
• The first feedback circuit may most preferably be connected to a current sensing point between said first and second current sensing resistors to determine said first feedback signal, i.e. corresponding to the momentary lamp current.
• Most preferably, the second feedback circuit is connected with said second supply terminal of said input device to determine a second feedback signal, corresponding to the momentary global current.
• According to a further development of the invention, the second output terminal of the power shaping circuit is connected to the reference potential, e.g. ground potential. Additionally or alternatively, the first current sensing resistor is connected in series between said second input terminal and said second output terminal of the power shaping circuit, i.e. in the second supply connection. Alternatively or additionally, the second current sensing resistor may preferably be arranged in series between said second supply terminal and said second input terminal.
• Due to the resulting connection of the bleeder circuit to the second input terminal of the power-shaping circuit, i.e. “between” the sensing resistors, the current through and thus the voltage over the second sensing resistor, i.e. the resistor between the second supply terminal and the power-shaping circuit includes the current, drawn by the bleeder circuit. Accordingly, the voltage over the second sensing resistor corresponds to the momentary global current, while the voltage over the first sensing resistor corresponds to the momentary lamp current.
• From the above, it will become apparent that two possibilities exist with regard to the positioning of the first sensing resistor between the second input and output terminals. The resistor may be placed either between the first input terminal of the power shaping circuit and the second connection point of the damping circuit, i.e. “between” bleeder and damping circuit, or between the second connection point and the second output terminal. According to the above first alternative, any additional current drawn by the damping circuit does not influence the first feedback signal. However, by placing the resistor between damping circuit and output terminal, the first feedback signal corresponds to the combined current drawn by the lighting unit and the damping circuit. The latter is particularly advantageous when a relatively constant global current is desired. The specific arrangement certainly depends on the application.
• Preferably, the lamp driver unit is connected between said first output terminal of the power shaping circuit and said reference potential, e.g. ground potential.
• Most preferably, the input device comprises a full-bridge rectifier, wherein a positive output of said rectifier is connected to said first supply terminal and a negative output of said rectifier is connected to said second supply terminal.
• The present embodiment advantageously allows to operate the circuit arrangement directly with a sinusoidal or alternating mains voltage, resulting in an enhanced versatility.
• According to a further development of the invention, the damping circuit is adapted, so that upon detection of a dimmer edge, the global current is controlled to an increased edge current, higher than said predefined minimum load current. Preferably, the peak edge current is higher than 10% of the predefined minimum load current.
• Due to increased current at or shortly after the dimmer edge, e.g. approx. 200 microseconds after the edge, oscillations, caused by the dimmer edge are advantageously suppressed or at least substantially reduced. Such oscillations may cause the global current to drop significantly and in particular below the minimum hold current of the power supply, which should be avoided.
• For reasons of energy efficiency, the edge current should be applied during a short period only, i.e. an edge current pulse. Preferably, the edge current pulse has a duration of 100-500 μs FWHM, preferably between 150-300 μs FHWM.
• As discussed in the preceding, the damping circuit may comprise capacitive elements. To avoid that upon operation of the bleeder circuit, any capacitive elements of the damping circuit or the lamp driver unit are discharged, it is particularly preferred that a current flow restrictor is connected between the first input terminal and the first connection point of the damping circuit with the first supply connection, i.e. between the bleeder and damping circuits. The flow restrictor may be of any suitable type and preferably comprises at least a flow restricting diode.
• Most preferably, a second current flow restrictor may be connected between the connection point of the damping circuit and the first output terminal, i.e. the lamp driver, so that current flow from the lamp driver to the damping circuit is avoided.
• To obtain the above mentioned setpoint current for the controllable lamp current controller, it may be desired to set the current in dependence of a dim level of the phase-cut operating voltage, i.e. according to the dimmer knob setting of the connected phase-cut power supply. As will become apparent to one skilled in the art, in a phase-cut dimmer, the dim level corresponds to the conduction interval, i.e. time between the dimmer edge and the following zero-crossing of the phase-cut operating voltage, so that determination of the conduction interval in at least one half-cycle of the sinusoidal operating voltage allows to easily retrieve the dim setting.
• Accordingly and particularly preferred, the circuit arrangement further comprises a dim level detector, configured to determine a dim level from said phase-cut operating voltage, said dim level detector being connected with said lamp current controller to set the setpoint current in dependence of the determined dim level.
• The dim level detector may be of any suitable type and preferably comprises an edge detector and a zero-crossing detector, so that the setpoint current is controlled to correspond to the length of the conduction interval between the zero-crossing and the detected edge or between the edge and the subsequent zero-crossing. Alternatively or additionally, the dim level detector may be adapted to integrate the rectified operating voltage. In this case, the integrated operating voltage corresponds to the dim level.
• Most preferably, the dim level detector is integrated with the input device and/or the two-port power shaping circuit.
• A further aspect of the present invention relates to an LED lamp comprising a circuit arrangement corresponding to one or more of the above mentioned embodiments and at least one LED-unit connected to the circuit arrangement, i.e. to the lamp driver unit of the circuit arrangement. The LED unit here may be of any suitable type as mentioned in the proceeding. Most preferably, the at least one LED unit and the circuit arrangement are comprised in common housing of the LED lamp.
BRIEF DESCRIPTION OF THE DRAWINGS
• These and other aspects, features and advantages of the present invention will be apparent from and elucidated with reference to the description of preferred embodiments in conjunction with the enclosed figures, in which
• FIG. 1 illustrates a first embodiment of a circuit arrangement according to the invention in a schematic block diagram,
• FIG. 2 illustrates a second embodiment of the circuit arrangement in a schematic block diagram,
• FIG. 3 illustrates a third embodiment of the inventive circuit arrangement in a further schematic block diagram,
• FIG. 4 illustrates the embodiment ofFIG. 2 in a detailed circuit diagram and
• FIG. 5aillustrates a further embodiment of the circuit arrangement according to the invention in a schematic circuit diagram and
• FIG. 5bshows the embodiment ofFIG. 5ain a more detailed circuit diagram.
DESCRIPTION OF EMBODIMENTS
• FIG. 1 shows a first embodiment of theinventive circuit arrangement1 in a schematic block diagram. To facilitate understanding of the present invention,circuit arrangement1 inFIG. 1 is shown in an operational state, i.e. connected to a phase-cut power supply2 and twohigh voltage LEDs3. Thepower supply2 comprises a leading-edge (LE) dimmer4 connected with a power source,e.g. mains5. Thepower supply2 accordingly provides a phase-cut operating voltage to thecircuit arrangement1, i.e. an alternating voltage, where a part of each half cycle is chopped or cut out during a dimming operation ofdimmer4. Since dimmer4 is of LE type, the chopped part is the front part of the waveform of the operating voltage in each half cycle. TheLEDs3 emit white light and have a power consumption of approx. 9 W. A lamp housing with a typical socket connection (both not shown) is provided to accommodate thecircuit arrangement1 and theLEDs3.
• Circuit arrangement1 comprises aninput device6, a two-portpower shaping circuit7 and alamp driver unit8, which according toFIG. 1 are arranged in a cascaded, i.e. serial connection.
• Theinput device6 serves to connect the further components of thecircuit arrangement1 and theLEDs3 with thepower supply2 over a typical plug/socket connection (not shown) and thus to provide power. Theinput device6 according toFIG. 1 comprises a typical diode bridge rectifier9 adapted to provide a rectified phase-cut operating voltage between afirst supply terminal10aand asecond supply terminal10b. Thesupply terminals10a,10bare connected with thepower shaping circuit7, i.e. with first andsecond input terminals11a,11bof thepower shaping circuit7.
• Thepower shaping circuit7 comprises a first21aand a second21bsupply connection, which connect the first andsecond input terminals11a,11bwith first andsecond output terminals12a,12b, respectively. Thepower shaping circuit7 further comprises ableeder circuit13 and a dampingcircuit14, connected between thesupply connections21aand21b, the operation of which is explained in detail in the following.
• The first12aand a second12boutput terminals of thepower shaping circuit7 are connected with thelamp driver unit8 to provide operating power to the twoLEDs3. Thelamp driver unit8 according to the present embodiment is a tapped linear driver comprising controllablecurrent sources15a,15band15c. As can be seen, the controllablecurrent sources15aand15bare connected in series to theLEDs3 in a typical tapped linear driver configuration, so that theLEDs3 are subsequently provided with power when the applied voltage is high enough to set therespective LED3 to a conductive mode.
• Current source15cis connected in series with a “fill-in”-buffer capacitor16 to provide that the fill-incapacitor16 is maintained a suitable level.
• The “fill-in”capacitor16 allows to power theLEDs3 even when the voltage applied is lower than the voltage of one of theLEDs3, which assures at least one of theLEDs3 is provided with power over the entire half cycle of the phase-cut operating voltage and thus provides that the light output is substantially constant and does not show visible flicker.Switch17 allows to discharge the “fill-in”capacitor16 if necessary.
• Thelamp driver unit8 further comprises a lampcurrent controller19. Lampcurrent controller19 comprises acomparator22 and is connected to control the current sources15a-15candswitch17, as shown inFIG. 1 by the dashed lines. The lampcurrent controller19 serves to control the current(s) through theLEDs3. The lampcurrent controller19 is connected to afirst feedback circuit18 to receive a first feedback signal, corresponding to the momentary lamp current and to dimlevel detector20 to receive a dim signal, corresponding to the setting ofLE dimmer4. For reasons of clarity, the connection betweendim level detector20 and thecomparator22 of the lampcurrent controller19 is indicated by arrows.
• Thedim level detector20 is configured to derive the dim signal from the rectified phase-cut voltage and thus is connected tofirst supply connection21a.
• The lampcurrent controller19 compares the momentary lamp current, as provided byfirst feedback circuit18 and the dim signal to set the current sources15a-15cand thus the brightness of theLEDs3 to correspond to the dim signal. As will be apparent to one skilled in the art, the lampcurrent controller19 thus provides closed-loop operation to set the lamp current in accordance with the dim signal, i.e. a “setpoint current”, i.e. according to the present embodiment +−0.5 mA, ˜1% of the desired setting so as to provide that the lamp current closely “matches” the dim setting.
• As can be seen from the figure, thefirst feedback circuit18 is connected to thesecond supply connection21b, i.e. to a current sensing point between firstcurrent sensing resistor23 and secondcurrent sensing resistor24.
• Thepower shaping circuit7 comprises, as mentioned in the preceding,bleeder circuit13. Thebleeder circuit13 serves as a further current path between the first andsecond supply connection21a,21bto enhance the compatibility with typical dimmers, such asLE dimmer4. Since corresponding types of dimmers typically show a minimum load/hold current to keep the dimmer in a conductive state, thebleeder circuit13 draws current in addition to thelamp driver unit8 when the current drawn by thedriver unit8 is below a predefined minimum load current. This may be particularly the case at relatively low dimming levels due to the reduced current consumption.Bleeder circuit13 comprises a controllable current source (not shown) and is connected with asecond feedback circuit25, which is connected to thesecond supply terminal10bof theinput device6 to obtain a second feedback signal, corresponding to the overall, i.e. the momentary global current.Comparator26 serves to invert the polarity of the second feedback signal.
• Thepower shaping circuit7 further comprises dampingcircuit14. The dampingcircuit14 is configured to attenuate high frequency oscillations in said operating voltage by drawing additional current upon the detection of a dimmer edge, i.e. at approximately 200 μs thereafter. The dampingcircuit14 comprises a capacitor/resistor network (not shown) that is tuned to a resonance frequency of the dimmer in such a way, that the resistance of the network provided appropriate damping. To provide that thebleeder circuit13 and the dampingcircuit14 during operation draw additional current from thepower supply2 only,diodes28 are arranged in thefirst supply connection21a.
• Besides the before mentioned components,circuit arrangement1 further comprises alow voltage supply27 to provide power for thecircuit arrangement1 and in particular for the operation ofbleeder13,dim level detector20 and lampcurrent controller19.
• During operation, operating voltage is present at thesupply terminals10aand10b. A corresponding current flows through thepower shaping circuit7, thelamp driver unit8 and theLEDs3. It is noted, that thesecond output terminal12band the negative sides of the current sources15a-15care connected to ground potential. Accordingly, the tworesistors23,24, which form a voltage divider, provide that the voltage at thesecond supply terminal10bof theinput device6 is “shifted” versus ground potential. As will be apparent, the shift in the voltage is dependent on the respective current. While the voltage over the firstcurrent sensing resistor23 thus corresponds to the lamp current (and the current of damping circuit14), the voltage at thesecond supply terminal10bcorresponds to the global current, i.e. including the current drawn by thebleeder circuit13 and dampingcircuit14.
• Accordingly, the first feedback signal offirst feedback circuit18 corresponds to the momentary lamp current and the second feedback signal of thesecond feedback circuit25 corresponds to the global current. The present embodiment ofcircuit arrangement1 thus allows to determine both, the lamp current and the global current simultaneously with a cost-effective circuit design and in particular without the losses of a current measurement in series with theLEDs3, i.e. in the part of thecircuit arrangement1, where relatively high currents are present during operation.
• FIG. 2 shows a second embodiment of theinventive circuit arrangement1′. The embodiment ofFIG. 2 corresponds in general to the embodiment ofFIG. 1 with the exception ofpower shaping circuit7′ and thelamp driver unit8′.
• According to the present embodiment,lamp driver unit8′ comprises twocapacitors30 in parallel to theLEDs3 instead of the aforementioned “fill-in”capacitor16 and the combination ofcurrent source15candswitch17, thus providing a reduced complexity of the setup ofcircuit arrangement1′. Thecapacitors30 according to the present embodiment serve to as energy storage or buffer, in case the provided voltage is too low to power theLEDs7, i.e. close to the zero-crossing in each half cycle of the alternating phase-cut operating voltage.
• Power shaping circuit7′ comprisesbleeder circuit13 and dampingcircuit14, however here, dampingcircuit14 is directly connected to thesecond output terminal12band thus ground potential. The changed setup provides that any current drawn by the dampingcircuit14 is advantageously included in the first feedback signal offeedback circuit18. Accordingly, it is possible to more precisely control the global current to the predefined minimum load current (at about 1% flatness).
• A third embodiment ofcircuit arrangement1″ is shown inFIG. 3. The embodiment corresponds to the embodiment ofFIG. 2 with the exception that the setup of thelamp driver unit8″ corresponds to a switching mode power supply (SMPS) with aMOSFET switch41. Further,power shaping circuit7″ comprises twodiodes28 in thefirst supply connection21a, according to the embodiment ofFIG. 1. As shown, the setup ofdriver unit8″ corresponds to a buck boost SMPS. Thedriver unit8″ further comprises an EMI-filter40 so that the high-frequency switching ofswitch41 does not interfere with the operation ofbleeder circuit13 and dampingcircuit14.
• FIG. 4 illustrates the embodiment ofFIG. 2 in a detailed circuit diagram, but without thepower supply2. As can be seen, thebleeder circuit13 comprises a current source with a FET, operated in linear mode. In this example, the current source is controlled between two levels, dependent on the rectified mains voltage. Accordingly, a low ohmic path is realized during the OFF state of thedimmer4.
• Thesecond feedback circuit25 is connected to secondcurrent sensing resistor24 and is furthermore connected to a low voltage transistor of thebleeder circuit13.
• Thecircuit arrangement1′ further comprisesfirst feedback circuit18, which is realized using an OP-AMP over said firstcurrent sensing resistor23. At the input of the OP-AMP, a clamping diode is arranged to prevent negative voltages. The current through thesensing resistor23 is clamped to a predefined reference value.
• Thelamp driver unit8′ comprises a power stage based on a tapped liner driver, as mentioned in the preceding, and comprises twohigh voltage LEDs3 or two strings ofLEDs3. EachLED3 has a correspondingelectrolytic capacitor30 in parallel. The shown two controllablecurrent sources15a,15bare attached to eachLED3 providing power in dependence of the input voltage saidcurrent sources15a,15bswitch. The actual amplitude during the mains cycle of the current sources is controlled by said OP-AMP.
• Thedim level detector20 provides a reference voltage as a function of the phase-cut angle of thedimmer4, i.e. the rectified mains.Low voltage supply27 provides a constant low voltage of 12V to power thecircuit arrangement1′. The dampingcircuit14 is formed as a passive R-C latch.
• FIGS. 5aand5billustrate a further embodiment of thecircuit arrangement1′″ according to the invention in a further detailed circuit diagram. The present embodiment shows a setup of acircuit arrangement1′″ in a switch mode power supply buck-boost configuration, but withoutLEDs3. Thecircuit arrangement1′″ comprises a non-dimmable IC AP1682 from BCD and a MOSFET switch. This embodiment comprises the building blocks described above, as can be seen fromFIGS. 5aand5b. For said first andsecond feedback circuits18,25, two OP-AMPs have been used.
• While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. For example, it may be possible to operate the invention in an embodiment in which:
• • a different number ofLEDs3 is present,
  • theLEDs3 have a higher or lower power,
  • thecircuit arrangement1,1′,1″,1′″ comprises circuitry to improve/flatten line regulation, e.g. by using further feedback loops,
  • thecircuit arrangement1,1′,1″,1′″ comprises over power and/or temperature protection circuitry,
  • thefirst feedback circuit18 and/or thesecond feedback circuit25 are formed as part of an integrated circuit and/or
  • in the embodiment ofFIG. 3, instead of a buck boost converter setup, a buck, a tapped buck, fly-back or half bridge setup is used.
• Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.

Claims (15)

1. Circuit arrangement for operating at least one low-power lighting unit with a phase-cut operating voltage from a power supply, comprising

an input device for connection to said power supply having at least a first and second supply terminal,

a two-port power shaping circuit, with at least

a first and a second input terminal, connected with the respective supply terminals of said input device,

a first and second output terminal, connected with said input terminals over first and second supply connections,

a controllable bleeder circuit, connected with said first and second input terminals to provide an alternative current path between said input terminals and being configured to set a global current, drawn during operation from the power supply to a predefined minimum load current, and

a damping circuit, connected to said first and second supply connections at a first and a second connection point to attenuate high frequency oscillations in said operating voltage, and

a lamp driver unit, connected to at least one of the output terminals of the power shaping circuit and being configured for connection to said at least one low-power lighting unit, said lamp driver unit comprising at least

a lamp current controller, configured to set a lamp current of said at least one low-power lighting unit and

a first feedback circuit, configured to provide a first feedback signal, corresponding to the momentary lamp current of said at least one low-power lighting unit, wherein the lamp current controller is connected with said first feedback circuit to control the lamp current in dependence of said feedback signal, so that the lamp current corresponds to a setpoint current, wherein the controllable bleeder circuit is configured to set the global current to the predefine minimum load current independently of the control of the lamp current.

2. Circuit arrangement according toclaim 1, wherein said controllable bleeder circuit is activated when the global current is lower than said predefined minimum load current.

3. Circuit arrangement according toclaim 1, further comprising a second feedback circuit, configured to determine a second feedback signal, corresponding to the global current and to provide said second feedback signal to the controllable bleeder circuit.

4. Circuit arrangement according toclaim 1, wherein said first and/or second feedback circuit is coupled to a series connection of a first and a second current sensing resistor, said series connection being coupled between said second supply terminal and a reference potential.

5. Circuit arrangement according toclaim 4, wherein said first feedback circuit is connected to a current sensing point between said first and second current sensing resistors to determine said first feedback signal.

6. Circuit arrangement according toclaim 3, wherein the second feedback circuit is connected with said second supply terminal of said input device to determine said second feedback signal, corresponding to the momentary global current.

7. Circuit arrangement according toclaim 4, wherein said second output terminal of said power shaping circuit is connected to said reference potential, said first current sensing resistor is connected in series between said second input terminal and said second output terminal, and said second current sensing resistor is arranged in series between said second supply terminal and said second input terminal.

8. Circuit arrangement according toclaim 1, wherein the lamp driver unit is connected between said first output terminal of the power shaping circuit and said reference potential.

9. Circuit arrangement according toclaim 1, wherein the input device comprises a full-bridge rectifier, wherein a positive output of said rectifier is connected to said first supply terminal and a negative output of said rectifier is connected to said second supply terminal.

10. Circuit arrangement according toclaim 1, wherein said damping circuit is adapted, so that upon detection of a dimmer edge, the global current is controlled to an increased edge current, higher than said predefined minimum load current.

11. Circuit arrangement according toclaim 1, wherein at least a current flow restrictor is provided in said first supply connection between said first input terminal and said first connection point, so that current flow from the damping circuit to the bleeder circuit is reduced.

12. Circuit arrangement according toclaim 1, further comprising a dim level detector, configured to determine a dim level from said phase-cut operating voltage, said dim level detector being connected with said lamp current controller to set the setpoint current in dependence of the determined dim level.

13. LED lamp comprising a circuit arrangement according toclaim 1 and at least one LED unit, connected to said lamp driver unit.

14. Lighting system comprising a LED lamp according toclaim 13 and a power supply to provide said LED lamp with a phase-cut operating voltage.

15. Method of operating at least one low-power lighting unit with a circuit arrangement, comprising

receiving by an input device a phase-cut operating voltage from a power supply having at least a first and second supply terminal,

connecting at least a first and a second input terminal of a two-port power shaping circuit with the respective supply terminals of said input device,

connecting a first and second output terminal with said input terminals over first and second supply connections,

connecting a controllable bleeder circuit with said first and second input terminals to provide an alternative current path between said input terminals, and a damping circuit, connected to said first and second supply connections to attenuate high frequency oscillations in said operating voltage, and

connecting a lamp driver unit to at least one of the output terminals of the power shaping circuit and being configured for connection to said at least one low-power lighting unit, wherein

the bleeder circuit sets a global current, drawn during operation from the power supply to a predefined minimum load current and

the lamp driver unit controls a lamp current in dependence of a first feedback signal, so that the lamp current corresponds to a setpoint current, wherein the bleeder circuit is configured to set the global current to the predefined minimum load current independently of the control of the lamp current.

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